In digital mobile communication systems, such as the GSM system, it is possible to send messages without setting up a connection with a circuit coupling. Such non-connection methods for message transmission include the short message service SMS, the unstructured supplementary service data USSD, or the general packet radio service GPRS. Thus, in addition to calls and data transmission, the GSM system offers a paging system type service in the form of the short message service. However, in view of an ordinary paging system, the short message system known from the GSM system is considerably more sophisticated. A mobile station can be used not only to receive text messages but also to transmit them to another mobile station. Another advantage of the short message service of the GSM system is that the transmission or reception of a short message can take place simultaneously when an ordinary circuit coupled connection is open, e.g. during a call. Thus, the transmission of a short message will not keep the mobile station busy upon a possible incoming call.
It is an advantage of short messages to calls that they can be forwarded to the receiver even if a connection could not be made to the receiver at the moment of transmission. This is implemented by dividing the transmission of a short message from a mobile station to another in two parts, as illustrated in FIG. 1: from the transmitting mobile station MS1 to a short message service centre SM-SC where the short message is stored and transmitted further to the actual destination, i.e. the receiving mobile station MS2, when a connection can be made to it. FIG. 2 illustrates the connection between the short message service centre SM-SC to the mobile communication system in more detail. In the following, the transmission and progression of short messages between different interfaces, known as such, will be described with reference to FIGS. 1 and 2.
FIG. 2 shows the structure of the mobile communication system and the connections for transmitting short messages. Mobile stations MS have radio communication with base transceiver stations BTS. The base transceiver stations BTS communicate further, via a so-called Abis interface, with a base stations controller BSC which controls several base transceiver stations. A unit consisting of several base transceiver stations BTS (typically some dozens of base transceiver stations) and one base station controller BSC controlling them is called a base station subsystem BSS. In particular, the base station controller BSC controls the channels of radio communication and hand-overs between channels. On the other hand, the base station controller BSC communicates, via a so-called A interface, a mobile services switching centre (MSC) which coordinates the setting up of connections from and to mobile stations. Via the mobile services switching centre MSC, a connection is further made outside the mobile communication network. Said short message service centre SM-SC is coupled with the mobile services switching centre MSC.
For transmitting a short message from a mobile station MS1 (FIG. 1), the message to be transmitted is formed, and the telephone number of the mobile station MS2, i.e. the identification where the transmission is addressed, is given. The short message is usually entered via the keypad of the mobile station, and the telephone number can be entered via the keypad or selected e.g. from a telephone directory stored in the mobile station. Further, the mobile station transmitting the short message must know the contact data, i.e. the telephone number, of the short message services centre SM-SC. This is usually stored in the memory of the mobile station, wherein there is no need to enter it separately in connection with each short message transmission. When the short message is transmitted, it passes from the mobile station MS to the base transceiver station BTS and further via the base station controller BSC and the mobile services switching centre MSC to the short message services centre SM-SC. The short message is stored at the short message services centre SM-SC, from where it is transmitted further to the receiving mobile station MS2, wherein the route is the same as in the transmission but reverse. The short messages services centre SM-SC is informed whether the mobile station MS2 has received the short message. Thus, it can retransmit the short message, if it was for any reason not received by the mobile station MS2.
In the short message service SMS of the GSM system, it is possible to transmit a message of a maximum length of 160 characters at a time. The characters are e.g. ASCII characters (American National Standard Code for Information Interchange) of 7 bits, but it is also possible to transmit characters of 8 bits which are not necessarily ASCII characters. Also short message services have been developed where the message can be transmitted in several short messages of a maximum length of 160 characters, whereby the length of the short message can be multiplied. For example in the GSM mobile communication system, the abbreviation C-SMS is used for this concatenated short message service.
The general packet radio service GPRS comprises separate network elements, such as GPRS support nodes and GPRS registers, as well as transmission systems. The transmission system used can be for example the TCP/IP (Transmission Control Protocol/Internet Protocol) network. Because the data transmission is conducted in a packet network where the data to be transmitted is conveyed in one or several data packets, the maximum benefit is obtained by all service subscribers from such packet form data transmission. In such packet data transmission, the data transmission channel (radio channel) is not reserved for one user during the entire connection, but only when data packets are being transmitted. This makes it possible to transmit data packets of several users in the same data transmission channel so that the data packets of different users are transmitted in their own time slots. Each data packet is equipped with address data, i.e. the telephone number or another identification of the recipient. In this connection, reference is made e.g. to the standard GSM 03.60 of the European Telecommunication Standards Institute ETSI, containing a more detailed description of the general packet radio service GPRS of the GSM system.
In addition to data and voice transmission services transmitted via the communication channel, cellular systems offer supplementary services, whereby data transmission relating to these is conveyed via a signalling channel. Thus, data transmission relating to supplementary services can, if necessary, take place simultaneously with the connection on the communication channel.
Messages relating to supplementary services are transmitted between the mobile station on one hand and the home location register (HLR) or the visitor public land mobile network (VPLMN) of the cellular system on the other hand. The message to be transmitted contains information on how the network element should process the message. For supplementary services to be introduced in cellular systems, the messages related to the supplementary service can be transmitted e.g. in the GSM system as an unstructured supplementary service, i.e. USS data, wherein messages can be transmitted in a transparent manner also in such parts of the system that do not know said supplementary service. In this context, reference is made to the ETSI standard GSM 03.90 which contains a description on the transmission of unstructured supplementary service data USSD, applied in the GSM mobile communication network.